Pyridinium p-toluenesulfonate (9 mg, 35.4 μmol) was added to a solution of (8E,12E,14E)-7-acetoxy-3,6,21-trihydroxy-6,10,12,16,20-pentamethyl-18,19-epoxytricosa-8,12,14-trien-11-olide (200 mg, 0.354 mmol) in dichloromethane (10 mL) and ethyl vinyl ether (521 mg, 7.08 mmol) at room temperature, followed by stirring at the same temperature overnight. Ethyl acetate and water were added to the reaction solution, and the mixture was extracted with ethyl acetate, dried over anhydrous magnesium sulfate, filtered and then evaporated. The resulting residue was purified by silica gel column chromatography (Kanto silica gel 60N, 40 to 50 μm; ethyl acetate-hexane=1:4, 1:3 and 1:1, successively) to obtain the title compound (250 mg, 94%) as a colorless oil.
